The present invention relates to a method and system for determining the efficiency of a catalytic converter based on signals generated by pre-catalyst and post-catalyst exhaust gas oxygen sensors.
As is known in the art, increasingly stringent federal regulations limit the permissible levels for emissions. As such, vehicle manufacturers have developed various methods to reduce emissions while improving vehicle performance and fuel economy. Catalytic converters, positioned in the engine exhaust path, are often used to reduce emission levels of regulated exhaust gases. The conversion efficiency of a catalytic converter may be monitored using a pre-catalyst O2 sensor positioned upstream from the catalytic converter and a post-catalyst O2 sensor positioned downstream from the catalytic converter.
One method known for indicating conversion efficiency of the catalyst is to calculate a ratio of downstream sensor transitions or switches to upstream sensor transitions or switches. An increasing switch ratio is generally indicative of a degrading catalyst. When the switch ratio exceeds a threshold value, a malfunction indicator light (MIL) is illuminated so the vehicle operator will seek service. This method of catalyst monitoring is disclosed in Orzel U.S. Pat. No. 5,357,751, assigned to the assignee of the present invention, and is referred to as the Switch Ratio (SR) method. Another method for indicating conversion efficiency of the catalyst is based on the ratio of the arc lengths of the downstream sensor signal to the arc lengths of the upstream sensor signals identified as an Index Ratio (IR) method in contrast to the SR method. This method is disclosed in U.S. Pat. No. 5,899,062 Jerger et al, assigned to the assignee of the present invention, and incorporated herein by reference.
The test cycle for catalyst monitoring requires collection of data from each of the sensors while the engine is operating in each of a plurality of inducted airflow ranges or air mass (AM) cells. In each method a predetermined number of transitions or switches of the upstream sensor in each AM cell is required to complete the test cycle. These methods rely on AM cell calibration and assume that sensor signal transitions occurring in a defined AM cell are valid for ratio computation regardless of engine speed and load conditions. The determination of SR and IR based on data taken while the driver is operating the vehicle at a high load, low rpm or low load, high rpm condition results in increased SR and IR variability even though operation is within one of the plurality of inducted airflow ranges. The determination of catalyst conversion efficiency based solely on AM conditions may result in error, and may reduce the ability to discriminate between a good and a failed catalyst.
A method of detecting catalytic converter deterioration based on the ratio of the arc length or the number of transitions of signals from sensors upstream and downstream of the converter where ratio determination is restricted to predefined air mass ranges within corresponding predefined engine speed/load ranges in order to avoid areas of engine speed and load instability that might impair test to test repeatability of the deterioration detection is described in U.S. Pat. No. 6,195,986 assigned to the assignee of the present invention, and incorporated herein by reference.
The inventors have recognized that all presently known methods are designed to monitor total oxygen storage degradation using different upstream to downstream O2 sensor signal calculations. Reference is made to co-pending patent application Ser. No. 10/062384, filed Feb. 1, 2002, inventors Nader et al., assigned to the same assignee as the present invention, the entire subject matter thereof being incorporated herein by reference. As described therein, oxygen storage can be found in two different catalyst wash coat components: Ceria (cerium oxides) and precious metal. Total oxygen storage availability is a function of the Ceria and precious metal content in wash coat and as well their dispersions and mutual locations (within the wash coat). The high O2 storage (Ceria) catalyst has been the standard for monitoring starting in circa 1994 model years. Ceria is the weaker link in the wash coat when compared to the precious metal (PM). Ceria degrades sooner than does the PM when exposed to thermal or chemical (phosphorus) degrading. The current production Index Ratio (IR) catalyst monitor measures the change in the lean O2 sensor signal amplitude, as the catalyst ages. The rear O2 sensor signal increases in activity as the catalyst loses ability to store oxygen. The Index monitor measures the catalyst O2 storage (ceria) only and infers the emissions. The ratio of the rear O2 sensor is compared to the front O2 sensor, as the ratio approaches 1.0 the catalyst failed.
During the catalyst monitor calibration process, the emission and catalyst index relationship are established testing differently aged catalyst. The index vs. Tail Pipe FTP emission function typically referred as a xe2x80x9chockey stick curvexe2x80x9d. Monitoring the index in the field alloys to infer the catalyst xe2x80x9chealthxe2x80x9d or tailpipe emission from catalyst index. The xe2x80x9cslopexe2x80x9d of the hockey stick curve is attributed to the loss of O2 storage that is measured as an increase in the amplitude of the rear, or CMS O2 sensor signal compared to the amplitude of front O2 sensor signal. At the knee of the curve, basically all of cerium oxides (oxygen storage) are gone. After the knee the emissions are still increasing but the Index ratio is constant. This flat portion of the xe2x80x9chockey curvexe2x80x9d is insensitive to the existing catalyst monitor. The catalyst has lost it""s ability to storage oxygen (high Index ratio) yet has good emissions. The cerium oxides based oxygen storage is gone (very susceptible phosphorus contamination) while the precious metal based oxygen storage still stay untracked. This catalyst was phosphorous poisoned in the field and turned on the malfunction indicator light (MIL). The concern is that while the catalyst has lost it""s ability to storage oxygen it may still be a very good emission catalyst. No longer is catalyst monitoring limited to a single measurement of O2 storage. At the knee of the xe2x80x9chockey stick curvexe2x80x9d the A/F signal amplitude entering and exiting the catalyst is almost the same. From this time on, the catalyst has no Ceria based O2 storage. The precious metal(s) (PM) alone continues to degrade but still carry some O2 storage due to affinity of oxygen and PM. The PM crystals grow larger (degrade) due to thermal aging thus reducing the active surface area and increase the emissions. The exhaust gas residency time or presence next to the active PM sites with some oxygen storage is the detectable (measured) metric, here time delay, xcfx84 (tau) or phase shift through the catalyst. On the flat portion of the hockey stick curve xcfx84 (tau) is still changing. xcfx84 (tau) is the transport delay (time) between the front and rear O2 signals which measures the change of catalyst activity. As the (no Ce O2 storage) catalyst degrades the value of xcfx84 (tau) decreases. The measurable O2 sensor signal change for a catalyst as it ages to low or no cerium oxides based oxygen storage is the time constant xcfx84 (tau). Tau is the time, or transport, delay between the upstream and downstream) O2 sensor signals. Tau or time delay varies at different rpm, loads, air mass and monitor volume for a given aged catalyst. However, xcfx84 decreases over time as the catalyst ages.
To put it another way, the inventors have recognized that the are two different types of material in the converter: one highly oxidizable (e.g., Ceria); and the other relatively less oxidizable. Thus, while increases in the amplitude of the oxygen sensed by the downstream converter indicates deterioration in the oxidizable material, and therefore its loss of effectiveness, there may still be effectiveness in the less oxidizable material performing the requisite emission reductions.
Applicants further recognized that the effectiveness of the less oxidizable material may be measured by measuring the time delay, or phase shift, between the signals produced by the upstream and downstream sensors. Thus, the applicants have determined that the effectiveness of each of the oxidizable and relatively non-oxidizable materials in the catalytic converter (i.e., the materials making up the catalyst) can be separately evaluated (i.e., measured independently); the former by the relative amplitudes between the oxygen before and after the catalyst; and, the latter by some other measurable parameter, such as time delay through the catalyst.
Further, the inventors have discovered that by measuring the time delay between the upstream and downstream oxygen sensors when such sensors transition from a lean condition to a rich condition, as distinguished from transitions from a rich condition to a lean condition, the reliability of the measurement of catalyst efficiency is improved.
In accordance with the present invention a method is provided for determining the effectiveness of a catalyst to remove emissions from such exhaust. The method includes measuring an upstream time history of a signal produced by an exhaust gas oxygen sensor upstream of the converter and a time history of an exhaust gas oxygen sensor disposed downstream of the converter. The time histories vary between a lean air-fuel ratio signal level and a rich air-fuel ratio signal level. A time delay is determined between the upstream time history and the downstream time history during transitions in such time histories from the lean air-fuel ratio signal level to the rich air-fuel ratio signal level. The determined time delay is compared with a reference time delay to determine the efficiency of the converter. The converter is determined to be ineffective if the converter as been determined to be potentially ineffective and the time delay is determined to be less than the reference time delay.
The details of one or more embodiments of the invention are set forth in the accompadrawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.